That's why they are dumping the advertising on his driveway, and why newspapers often provided a separate box for your newspaper that you mount below or beside your mailbox. I used past tense because we have no local newspaper anymore.
If they put this crap in your mailbox, they'd be committing a federal crime and postal inspectors would probably track them down if they got enough complaints.
how does this work with outgoing mail?
>statute that prohibited the deposit of unstamped “mailable matter” in a mailbox
I assume the outgoing mail, if stamped, is ok. Edit; UPS may even be able to use it if the item has postage paid, but in that case they would just have USPS deliver it.
A more complete (but still not fully so) statement is that it is generally illegal for anyone but you and the postal service to put anything into your mailbox, and it is also generally illegal for anyone but you and the postal service to take anything out.
I saw one as I was leaving one day who had just left a typical pamphlet in our mailbox. I pointed out the no junk unsolicited sign and he started on how it was important information I may be interested in.
I suggested that while that was a nice rationalisation, he knows it's bullshit and he should get his hand off it.